Effluents from wastewater treatment plants pose environmental hazard to the receiving water bodies mainly due to the contents of carbon, phosphorus and nitrogen (C, P and N), particularly if the plant is not designed to perform tertiary nutrient (N, P) treatment. These nutrients are the major stimulants of eutrophication and they should be eliminated from the effluent before discharge into the aquatic environment. Currently, new treatment facilities are designed to remove these nutrients to extremely low levels as a part of sustainable water management. The ever more stringent regulations require the retrofitting of the existing wastewater treatment plants to meet the disposal requirements and reduce the concentration of these nutrients as much as possible. In conventional treatment plants, the removal of C, P and N requires several biological reactors or zones within one reactor working simultaneously at different operating conditions to create the optimum environment for the removal of each individual nutrient.
The aerobic activated sludge reactor is by far the most widely applied method to remove carbon (C) through the oxidation of the organic materials by the microbial biomass. Phosphorus (P) removal involves the recycling of biomass into anaerobic and aerobic zones in order to promote the accumulation of phosphate by micro-organisms in a process known as enhanced biological phosphorus removal (EBPR). Biological P-removal can produce an effluent with soluble P as low as about 0.2 mg/L although designers assume EBPR removals only to 0.5 mg/L. Chemicals such as aluminum sulfate and ferric chloride are common P precipitants that are used as alternatives to the EBPR process or in cases where lower P concentrations are demanded. On the other hand, Nitrogen (N) removal involves sequential aerobic and anoxic biological reactions to achieve complete transformation of the influent ammonium into nitrogen gas. Carbon source is added into the anoxic reactor to sustain the heterotrophic denitrifiers responsible for conversion of nitrate into gas, which is costly. The elimination of all these nutrients in one single reactor is a challenging task.